1. Field of the Invention
The present invention relates to a power transmission system in a vehicle, comprising a continuously variable gear path and a multi-stage gear path provided in parallel for transmitting a driving force from an engine to driven wheels.
2. Description of the Related Art
A conventional power transmission system in a vehicle is known from Japanese Patent Publication No. 63-14228.
In the conventional power transmission system, the rotation of a main shaft connected to an engine through a torque converter is transmitted to a counter shaft through a reduction gear train or a belt-type continuously variable transmission, and a forward/backward travel switching mechanism using a single-pinion type planetary gear mechanism is disposed between the counter shaft and driven wheels. At the start of forward travel of the vehicle and at the start of backward travel of the vehicle, a reduction gear train having a large reduction ratio is used; and during subsequent traveling of the vehicle, the reduction gear train is switched over to the belt-type continuously variable transmission to conduct the continuously variable gear shifting. The forward/backward travel switching mechanism for switching the forward and backward travels from one to another, includes a forward clutch adapted to couple a carrier to an output shaft, and a reverse brake adapted to couple the carrier to a casing.
The conventional power transmission system is designed as follows: when the forward clutch is brought into an engaged state, the carrier is coupled to the output shaft, whereby the rotation of the input shaft (the counter shaft) is transmitted, at the same speed and in the same rotational direction, to the output shaft to establish a forward gear shift stage; and when the reverse brake is brought into the engaged state, the rotation of the input shaft (the counter shaft) is transmitted, at an increased speed and in the reverse rotational direction, to the output shaft to establish a backward gear shift stage.
In this way, if the driving force is input through a ring gear into the forward/backward travel switching mechanism and output through a sun gear from the mechanism, during backward traveling of the vehicle, the rotational speed of the input shaft is increased and transmitted to the output shaft. Therefore, a problem exists that the driving force at the start of the backward travel is insufficient. Whereas, if the driving force is input through a sun gear into the forward/backward travel switching mechanism and output through a ring gear from the mechanism, the rotational speed of the input shaft is reduced and transmitted to the output shaft. Therefore, a problem exists that the driving force at the start of the backward travel is excessive as compared with that at the start of the forward travel.
Also, if a double-pinion type planetary gear mechanism is used for the forward/backward travel switching mechanism, a gear ratio between the input shaft and the output shaft can be set at 1:1, but resulting in several problems. The double-pinion type planetary gear mechanism having a large number of parts complicates the structure, and also the number of meshed portions of each gear is large and the rotational speed of the planetary gear is high resulting in an increase in noise.
The present invention has been accomplished with such circumstance in view, and it is an object to provide a power transmission system in a vehicle including a continuously variable gear path and a multi-stage gear path, in which a difference between gear ratios at the start of the forward travel and at the start of the backward travel is minimized while using a single-pinion type planetary gear mechanism.